1. Field of the Invention
The present invention relates to a focusing device and a focusing method that are used in an optical system mounted in a measuring and inspecting apparatus for semiconductor patterns of a semiconductor wafer, a photo mask and the like, and more particularly, to a technique for detecting precise focus even in the case where a line width or defect in a semiconductor pattern is not greater than the wavelength of an illuminating light used for focus detection.
2. Description of the Related Art
A measuring and inspecting apparatus for semiconductor patterns of a semiconductor wafer, a photo mask and the like is required to measure the line width of a pattern highly accurately by using an optical image and to detect even slight defects in a pattern with high resolving power.
Such a semiconductor pattern measuring and inspecting apparatus forms an enlarged optical image of a semiconductor pattern on a sensor, such as a charge coupled device (CCD). An electric image signal obtained by the sensor is processed for measurement and inspection. Highly accurate measurement and inspection require the formation of an optical image at high magnification and resolution. Accordingly, the optical image must be precisely focused.
FIG. 15 shows a focusing mechanism 100 that employs an optical lever method. In FIG. 15, reference symbol W denotes a subject; reference numeral 101 denotes an inspecting sensor; 102 denotes an objective lens; 103 denotes an imaging lens; 104 denotes an LD; 105 denotes a position sensor; 106 denotes a focus detecting circuit; and 107 denotes a focus control circuit. Using LD light, the focusing mechanism 100 forms an image of the focus position of the subject W on the position sensor 105, processes the signal of the position sensor 105, measures the degree of defocus, and controls the focusing.
Another focusing mechanism is known (refer to, for example, Jpn. Pat. Appln. KOKAI Publication No. 11-271597), which creates a front focus in which the focus is in front of the subject and a back focus in which the focus is behind the subject, and then determines the focus position on the basis of the difference between an integral value (i.e., contrast) of the light intensity of the image in the front focus state and that in the back focus state.
However, the aforesaid focus detecting method using the optical lever method has suffered from a drawback described below. That is, if a line width or pitch of a semiconductor pattern is not greater than the wavelength of an illuminating light for use in focus detection, light is diffracted by the semiconductor pattern and forms an image on the position sensor, with the result that a focus position may not be accurately detected. The latter focus detecting method, which uses integral values of light intensities, also has the drawback that if a difference between integral values is small, a focus position may not be accurately detected.